Re: RFC: Memory Tiering Kernel Interfaces

[Wei Xu]

On Wed, May 11, 2022 at 12:49 AM ying.huang@xxxxxxxxx
<ying.huang@xxxxxxxxx> wrote:
>
> On Tue, 2022-05-10 at 22:30 -0700, Wei Xu wrote:
> > On Tue, May 10, 2022 at 4:38 AM Aneesh Kumar K.V
> > <aneesh.kumar@xxxxxxxxxxxxx> wrote:
> > >
> > > Alistair Popple <apopple@xxxxxxxxxx> writes:
> > >
> > > > Wei Xu <weixugc@xxxxxxxxxx> writes:
> > > >
> > > > > On Thu, May 5, 2022 at 5:19 PM Alistair Popple <apopple@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > Wei Xu <weixugc@xxxxxxxxxx> writes:
> > > > > >
> > > > > > [...]
> > > > > >
> > > > > > > > >
> > > > > > > > >
> > > > > > > > > Tiering Hierarchy Initialization
> > > > > > > > > `=============================='
> > > > > > > > >
> > > > > > > > > By default, all memory nodes are in the top tier (N_TOPTIER_MEMORY).
> > > > > > > > >
> > > > > > > > > A device driver can remove its memory nodes from the top tier, e.g.
> > > > > > > > > a dax driver can remove PMEM nodes from the top tier.
> > > > > > > >
> > > > > > > > With the topology built by firmware we should not need this.
> > > > > >
> > > > > > I agree that in an ideal world the hierarchy should be built by firmware based
> > > > > > on something like the HMAT. But I also think being able to override this will be
> > > > > > useful in getting there. Therefore a way of overriding the generated hierarchy
> > > > > > would be good, either via sysfs or kernel boot parameter if we don't want to
> > > > > > commit to a particular user interface now.
> > > > > >
> > > > > > However I'm less sure letting device-drivers override this is a good idea. How
> > > > > > for example would a GPU driver make sure it's node is in the top tier? By moving
> > > > > > every node that the driver does not know about out of N_TOPTIER_MEMORY? That
> > > > > > could get messy if say there were two drivers both of which wanted their node to
> > > > > > be in the top tier.
> > > > >
> > > > > The suggestion is to allow a device driver to opt out its memory
> > > > > devices from the top-tier, not the other way around.
> > > >
> > > > So how would demotion work in the case of accelerators then? In that
> > > > case we would want GPU memory to demote to DRAM, but that won't happen
> > > > if both DRAM and GPU memory are in N_TOPTIER_MEMORY and it seems the
> > > > only override available with this proposal would move GPU memory into a
> > > > lower tier, which is the opposite of what's needed there.
> > >
> > > How about we do 3 tiers now. dax kmem devices can be registered to
> > > tier 3. By default all numa nodes can be registered at tier 2 and HBM or
> > > GPU can be enabled to register at tier 1. ?
> >
> > This makes sense.  I will send an updated RFC based on the discussions so far.
>
> Are these tier number fixed?  If so, it appears strange that the
> smallest tier number is 0 on some machines, but 1 on some other
> machines.

When the kernel is configured to allow 3 tiers, we can always show all
the 3 tiers. It is just that some tiers (e.g. tier 0) may be empty on
some machines.

BTW, the userspace should not assume a specific meaning of a
particular tier id because it can change depending on the number of
tiers that the kernel is configured with.  For example, the userspace
should not assume that tier-2 always means PMEM nodes.  In a system
with 4 tiers, PMEM nodes may be in tier-3, not tier-2.

> Best Regards,
> Huang, Ying
>
>